The invention generally relates to a disc refiner for lignocellulosic material such as wood chips. In particular, the invention relates to the grooves between adjacent refiner bars on opposing sides of plate segments mounted on the discs of the refiner or disparager.
Mechanical pulping involves mechanically separating fibers found in logs or wood chips or other lignocellulosic material. In some embodiments, such fibers may be suitable for paper making.
A common method of separating the fibers involves the use of a mechanical (or semi-chemical) refiner, which often consists of one rotating disc (e.g., a rotor) facing a stationary disc (e.g., a stator), with the rotating disc turning at speeds of approximately 900 to 2300 revolutions per minute (RPM), and in which wood material is fed into the center of the stationary disc. In some cases, both discs rotate in opposite directions, and in some other cases, there is a conical section following the flat disc surface. The discs are typically equipped with a number of replaceable segments (plate segments) positioned side by side and are mounted to the disc, these plate segments having an array of bars and grooves. The grooves generally have dams to impede the progression of wood material from the center of the plate segment inner edge to the outer edge of the plate segment. As the bars from the opposing discs cross, crossing bars impart compressive and shear forces to the lignocellulosic material. The compressive and shear forces cause separation of the larger pieces of wood material into individual fibers, development of the fibers and, to some degree, cutting of the fibers. Cutting of the fibers may not be desirable.
In the typical designs of refiner plates or plate segments (the terms plate and plate segment will be used interchangeably), both facing discs feature a certain depth of the grooves which is substantially similar on opposite plates or plate segments. The profile of the groove depths relative to the distance from the center of the disc is generally flat and planar, either substantially parallel with the top of the refining bars or at a slight deviation from parallel, such that the depth (i.e., distance from the top of the refining bar to the bottom of the groove) gradually reduces towards the periphery of the plate.
FIG. 1 illustrates a cross-sectional view of a set 100 of complementary conventional refiner plate segments 102 and 104. Unrefined lignocellulosic material 120, such as wood chip material, is fed near the conventional refiner plate inner edge 108. Refined lignocellulosic material 122 exits near the conventional refiner plate outer edge 106. Thus the material moves as illustrated in FIG. 1 from right to left. While moving as illustrated in FIG. 1, the material first encounters dams 130, 132, 134, 136, 138, and 140 in an innermost refining zone or a breaker bar zone 101. Conventional refiner plate segments 102 and 104 have a series of alternating bars 150, 152 and grooves (not shown). The tops of the bars 150, 152 of the respective conventional refiner plate segments 102, 104 face each other. As illustrated, bar 150 of conventional refiner plate segment 102 opposes bar 152 of conventional refiner plate segment 104.
Between bar 150 and bar 152 there is a gap 157 having a distance 164 between the tops of the bars. Gap 157 is generally uniform. In contrast the gap 162 between the bottoms of opposing grooves varies due to dams 154 and 156 in the grooves.
There are dams 154 and 156 on the respective conventional refiner plate segments 102, 104. These dams 154 and 156 force material traveling through the grooves defined by the respective surfaces 158 and 160 into the gap 157 and opposing conventional refiner plate segments 102, 104. As illustrated, the bottom of the opposing grooves has a distance 162. The distance between the bottom of the grooves and the tops of the respective dams, e.g. as illustrated by 166, varies along the radius illustrated in the cross section of FIG. 1. The number of dams, shape, distance, and height from the bottom of the grooves to the tops of the respective dams varies in different refiner plate designs of existing technology, based on the required retention of feed material.
Due to the centrifugal forces caused by the relative rotation of the discs, many refiner plate designs use dams in the grooves, which restrict the free flow of material in those grooves. These dams are believed to prevent unrefined material from flowing out of the discs without being mechanically treated.
Mechanical pulping can use significant amounts of energy and may produce large quantities of heat through dissipation of frictional energy. This heat transforms water from the process into steam; in most cases a substantial amount of steam is produced. The steam produced must evacuate from the refiner via the gap formed between the discs. Failure to evacuate this steam with relative ease is believed to cause mechanical vibration of the refiner, as well as process instability. In many instances, poor steam evacuation may also cause a limitation in the amount of energy that can be imparted to the lignocellulosic material, due to a limit of how much force the refiner can apply to hold the discs in close proximity to achieve the desired work. The steam may also travel together with the lignocellulosic material through the grooves in a non-rotating disc, and conventional stator refiner plates also include dams to prevent un-treated fibers from exiting the refining gap without mechanical treatment.
Refiner plates with various patterns of bars and grooves are known to those skilled in the art. See, e.g., U.S. Pat. No. 5,383,617 to Deuchars; U.S. Pat. No. 5,893,525 to Gingras; U.S. Pat. No. 6,032,888 to Deuchars; U.S. Pat. No. 6,402,071 to Gingras; U.S. Pat. No. 6,607,153 to Gingras; U.S. Pat. No. 6,616,078 to Gingras; and PCT Pub. No. WO/2010/112667 to Ruola et al.
The conventional bar, groove, and dam arrangements (e.g., as noted in the patents identified in the previous paragraph) may be effective at forcing material out of the grooves into the gap formed between the opposing discs, but the arrangements may restrict steam flow. It is believed that the path of steam flow through a refiner equipped with conventional refiner plates is turbulent (e.g., non-laminar) and may cause refiner instability. In addition, the very abrupt changes in groove depths due to dams and the short spacing between dams often results in steam flow being restricted to a very small percentage of the groove depths—thus limiting the steam evacuation efficiency.